Many types of implantable medical devices exist and have various functions. Once implanted, they must cooperate with an external element to continue their function. For example, a reservoir can be implanted to deliver a liquid composition at a given rate. The reservoir must be refilled at regular intervals, for example by means of a syringe. For implantable medical devices consuming electrical energy and that incorporate a rechargeable battery, such as neurostimulators, their batteries must be replenished at regular intervals. An implantable medical device may collect information from the implantable medical device itself (e.g., battery level of charge, logging information, physiological parameters) and from the subcutaneous environment surrounding it, and this information must be transferred to the exterior. An implantable medical device may need to be programmed or re-programmed after its implantation.
All these operations can easily be performed with an external element providing the type of cooperation required by a specific type of implantable medical device. In most cases, however, an optimal transcutaneous (wireless) cooperation between an implantable medical device and an external element requires an optimal positioning of the external element relative to the implantable medical device. For example, the intensity of an electromagnetic signal emitted by an emitter along a transmission axis drops rapidly with the radial distance from said transmission axis. Typically, a misalignment of the order of the mm may drop the strength of the reception of a signal by several tens of %. Similarly, the internal rechargeable battery of an implantable medical device can be recharged by inducing a current in a secondary coil of the implanted medical device by means of a magnetic field generated by current circulating in a primary coil in an external element. The intensity of the induced current in the secondary coil varies substantially as a function of the position of the primary coil with respect to the secondary coil.
WO2010042055 proposes to provide a system with a wireless feedback system arranged to transmit feedback information from a RFID transmitter. The feedback information thus transmitted is used to optimize the position of the primary coil of a charger relative a secondary coil of an implanted medical device.
WO9811942 describes an alignment circuit and indicator to indicate whether the coils of an external charger and of an implantable medical device are properly aligned. The alignment circuit senses current in the primary coil to determine whether the angular and lateral alignment is optimal by sensing a peak DC current.
WO2006012426 developed a system for locating an implanted object using an external antenna. WO2010042056 uses a system wherein the implanted medical device transmits feedback information to the external charger including the strength of the magnetic field generated by the primary coil.
WO2005000391 describes a sensor circuit arranged close to the primary coil so that a part of the magnetic field from the secondary coil is enclosed by the sensor circuit thereby providing an induction in the sensor circuit which can be detected and thereby indicates the condition in the secondary circuit.
The present invention proposes an original solution for the alignment of an external element relative to an implanted medical device. The system is simple, effective and cost effective. It can also be automated. These and other advantages of the present invention are presented in the next sections.